1. Field of the Invention
This invention relates to stage cementing of well casing and the like in subsea wells, especially oil or gas wells. More particularly, the invention pertains to methods of cementing wall casing in subsea and similarly situated wells. Also, the invention includes a subsea well installation for stage cementing well casing in a well. Further, the invention relates to a plug stack assembly for stage cementing a subsea well installation, and to the combination with a plug stack assembly of a protective crate therefor. The invention also relates to an improved trip-plug for operating a stage cementing collar to open the ports thereof. Still further, the invention relates to a dart adapted to enter the bore of a hollow plug to block the flow of liquid through the bore. The invention may also be employed on land based wells.
2. Description of the Prior Art
Heretofore, casing strings have been cemented in well bores in multiple stages. In multiple stage cementing, a first stage or first increment of cement slurry is pumped down the well casing string, out through first stage cementing ports adjacent the bottom of the string, and into the well bore. The slurry rises in the annular space between the casing and the well bore to a predetermined level, and is there maintained in a quiescent condition until it sets. A second stage or second increment of cement slurry is pumped down the casing, out through second stage cementing ports, and into the well bore at a level at or above the top of the column of first stage cement. The second stage of slurry rises in the annulus between the casing and the well bore to a second predetermined level, and is maintained quiescent while setting. Sometimes, a third or even a fourth stage of cement slurry is introduced into the annulus above a preceding stage. The cement bonds the casing to the walls of the well and prevents migration of fluids through the annulus.
Multiple stage cementing has many advantages over single stage cementing, in which but a single charge of cement slurry is deposited around the entire length of casing.
A multiple stage cementing operation reduces the likelihood of breaking down a weak earth formation with the high fluid pressures required to lift a long single column of cement slurry, thus minimizing the loss of slurry to thieving formations. Such a cementing procedure reduces the required pump pressures to magnitudes lower than those needed for a corresponding single stage job.
Multiple stage operations also reduce the length of travel of the slurry in contact with the earth formations surrounding the casing, to thereby reduce contamination of the slurry and insure the strength of the cement when it has cured.
Such multiple stage procedures reduce the quantity of cement required to cement widely separated intervals, as in dual zone wells.
Multiple stage cementing reduces channeling of the cement slurry into drilling mud in the annulus, thereby providing a stronger bond of the cement with the casing and the earth formations.
One previously known system for multiple stage cementing is described in Composite Catalog of Oil Field Equipment and Services, 31st Revision (1974-75), published by World Oil, a Gulf Publishing Company Publication, Houston, Texas, U.S.A., 1974, pages 334 to 341. Another such system is described in the foregoing Composite Catalog on pages 2434 to 2440. These known systems employ a casing string having a first-stage cementing port device disposed near the bottom of the string, and a stage collar disposed at an intermediate location in the string. First stage cementing plug structures are used in connection with the first stage cementing port device. Other plugs are used to open and close the ports of the stage cementing collar.
These known systems are practicable for use in cementing casing in the bores of land based wells, where the top of the casing is adjacent to the earth's surface, and the plugs can be launched directly into the top of the casing string. However, these systems are not easily adapted for use in cementing casing in marine based or subsea wells, where the top of the casing terminates at the ocean floor, which may be many hundreds of feet below the surface of the water at which the drilling vessel or platform is located. In those instances, the casing string has to be extended from the ocean floor up to the floating drilling vessel or platform through a riser pipe, so that the plugs can be launched into the casing. The modifications required for adapting these known systems to marine operations are time-consuming and costly.
A subsea well stage cementing system is disclosed in applicant's prior U.S. Pat. No. 3,730,267 issued May 1, 1973. In the system of that prior patent, the top of the casing string terminates at the ocean floor, and fluid connection to the floating or stationary platform at the surface of the water is established through a string of drill pipe. A stage cementing collar is located in the casing string at an intermediate point. A hollow top plug for the first stage of cement slurry is releasably positioned in the casing string below the stage cementing collar. The normally closed stage cementing collar is opened for the second stage cementing operation by dropping an opening-ball into the string of drill pipe and allowing it to drop to the stage collar, and thereafter applying fluid pressure in the casing string above the stage collar. A dart-actuated, hollow top plug for the second stage of cement slurry is releasably secured to a hollow mandrel adjacent to the top of the casing string. This dart-actuated top plug also serves to close the ports of the stage cementing collar after the second stage of cement slurry has been expelled into the annulus in the well bore.
A principal shortcoming of the system of the foregoing prior patent is that, prior to conducting the second stage of cementing, there is no provision for wiping the interior walls of the casing string above the stage collar to remove adherent cement slurry left thereon by the first stage of cement slurry. The time that elapses between the first and second stages of cementing may be such that the adherent cement slurry sets up on the interior walls of the casing, thereby to interfere with subsequent operations. Moreover, the first- and second-stage cementing plugs are mounted in the casing string at widely separated locations, which entails two plug-mounting steps. Also, due to the narrowness of the bore of the string of drill pipe, it is not practicable to drop a trip-plug therethrough to open the stage collar.